Sun, Sep 03, 2017 10:36 pm

I manage a Linux server that functions as an email server using the
free and open source software (FOSS) package
sendmail.
I provide a mechanism through the server for someone who has
Verizon as his
Internet Service Provider (ISP) to send monthly newsletters
by email to an organization that has about thirteen hundred members on its
email distribution list, since he can't send to that number of people through
his ISP-provided email service. I do so by providing an
email alias
on my server, e.g., thelist@example.com that he puts in the
BCC line of his email. The alias is stored in /etc/aliases
and points to a text file containing the list of all members' email addresses.
So his ISP-provided
SMTP server sees only the one address, thelist@example.com,
which results in an email message to the server I manage that then translates
that address into the approximately 1,300 email addresses of members and sends
the newsletter to all members.

But this month the user reported he had sent the
message, but it had not been delivered to recipients. I first checked the
server's mail log, /var/log/maillog, for any occurrences of his
email address for the day he reported the problem. I use several free
DNS-based
Blackhole List (DNSBL) services to reduce the amount of
spam that
reaches user's inboxes, so I suspected that one of those services had blocked
email from the SMTP server through which he was sending his message, even
though I had whitelisted his email address quite some time ago by
adding a line like the following one to /etc/mail/access and
then running the command makemap hash /etc/mail/access
</etc/mail/access.

slartibartfast123987@verizon.net OK

I didn't find any references to his email address in the /var/log/mail
file, so I asked him to resend the message. I still didn't see any
references to his email address in the /var/log/maillog file,
but I did see that SORBS had blocked email from an
America Online (AOL)
server at the time he sent the message.

Fri, Jun 02, 2017 10:44 pm

I was notified by someone today that yesterday he had sent an email to a mailing
list on an email server I maintain, but the email had not been delivered
to recipients. When I checked yesterday's email log, I didn't see any email
from his email address, so I asked him to resend the message. He did so, but
that email message was also not delivered and I didn't see any log entry for
his email address in today's email log, /var/log/maillog. He
has a verzion.net email address and Verizon recently transitioned its email
service to AOL.
I remembered helping him make that transition last month, so I looked for any
aol.com entries in the log file and found the entry below for an attempt by an
AOL email server to deliver a message that was rejected at the time he told me
he had sent the email today.

Sun, Apr 30, 2017 8:33 pm

A family member reported that she hadn't received an email message sent to
her today by a
Gmail user who had sent her message in reply to the family member's
email to her. Since I administer the
Sendmail
email server she uses, I checked the Sendmail log file at
/var/log/maillog. I saw the outgoing email sent to the Gmail
address, but no incoming email from that address. So I sent email
messages from a Gmail account I have as well as email messages from other
external addresses to the root account on the server. The other email
messages arrived, but none I sent from the Gmail account arrived. So I ran
tcpdump
on the server to capture data to/from port 25 on the system, which is the
well-known port for
Simple Mail Transfer Protocol (SMTP) traffic. I then sent
another email message to the root account on the Sendmail server from my
Gmail account. After allowing several minutes for an attempted delivery from
the Gmail server to occur, I stopped the packet capture with Ctrl-C.